An X-ray computed tomography apparatus irradiates a subject with X-rays using an X-ray tube, and detects X-rays transmitted through or scattered by the subject using a radiation detector. The X-ray computed tomography apparatus then acquires electrical signals from the radiation detector using a data acquisition system (DAS), and generates the data of a fluoroscopic image, tomographic image, or three-dimensional image of the subject based on the acquired electrical signals. As a DAS in a general X-ray computed tomography apparatus, a DAS in the integral mode is known.
In the integral mode, the DAS integrates electrical signals from the radiation detector for a predetermined period of time and outputs an integral signal. In the integral mode, the DAS integrates all the energies of incident X-rays, i.e., all the energies of X-rays from the low energy to the high energy. For this reason, the information of X-rays with relatively low energies becomes obscure. An image reconstructed by using such an integral signal contains artifacts due to the hardening of the quality of X-rays (a so-called beam hardening phenomenon) or decreases in the contrast resolution of a soft tissue, resulting in a deterioration in image quality.
Recently, in pursuit of new applications of an X-ray computed tomography apparatus, vigorous research and development efforts have been made on a photon count type CT which operates in the photon-counting mode. In the photon-counting mode, the DAS counts the electrical signals generated by making the radiation detector detect X-rays, and indirectly detects the count value as the number of photons of X-rays. The photon-counting mode eliminates the obscurity of low-energy information concerning the above integral mode. The photon-counting mode is lower in the maximum count rate indicating photon counting ability than the integral mode. From the viewpoint of such a photon count rate, it is difficult to apply the photon-counting mode to an X-ray computed tomography apparatus.
It is an object of an embodiment to provide an X-ray computed tomography apparatus and radiation detector which realize the photon-counting mode while maintaining high X-ray photon counting ability in the integral mode.